What is fastest, rendering on a CPU or a GPU?
Rendering is an extremely complicated process, thus the actual rendertime of a given scene depends on vastly more things than the actual hardware. Things such as scene size, shader complexity, and poly count (as well as dozens and dozens of other factors) all play into rendertimes.

Since there are literally hundreds of factors that affect rendertimes, it's really impossible to say how long a given scene would take to render.  Even if you knew all of the major factors it would still be virtually impossible to quantify these and say "it will take X hours to render, without actually rendering it.



That said, you can somewhat accurately generically compare the relative speed between two pieces of hardware.  So most of the time the gtx980ti will be considerably faster than most any CPU.

That said, I'd say save your money for now.  Your current specs are pretty darn fast, rendering is just a complicated process and will take time no matter what your specs are.  In the future I'd probably upgrade the GPU to an NVIDIA, but it won't be massively faster (unless you go for a Titan or m6000 or something like that). Compare your times to this article, it's a year old but still fairly relevant.

You may also want to consider going for two less powerful GPUs (provided your motherboard is SLI capable) and getting them one at a time.